1. Field of the Invention
This invention relates to a brake booster and more particularly to a vacuum type brake booster for automobiles. An interior of the brake booster housing is divided by a movable wall into a constant pressure chamber connected to a vacuum source, such as intake manifold, and a variable pressure chamber with a hub member connected to the wall. The hub member includes therein a control valve which is manually operated to interrupt the fluid communication between the two pressure chambers and at the same time to allow an ambient air flow into the variable chamber for generating the operational force of the movable wall. Such operational force of the movable wall is transmitted to an output of the booster via a reaction mechanism.
2. Prior Art
Conventional boosters of the above type generate operational force on the movable wall only by manually actuating the control valve to introduce the pressure into the variable pressure chamber. In order to avoid continual depression of the brake pedal during parking of the vehicle while maintaining the complete stopping of the vehicle particularly on a sloped road, or, when the vehicle transmission is in "D" position, several proposals have been made.
One example is shown in Japanese patent publication No. 130845/82 wherein instead of using manual operation of a rod for actuating the control valve, an hydraulic actuator or an electromagnet is placed outside the body or within the hub member for actuating the control valve to introduce the ambient air into the variable pressure chamber thereby to generate the operational force of the movable wall. In this structure, however, a need for sufficient output from the actuator or from the electromagnet device exists to overcome the reaction force of the booster, which results in a size increase of the overall booster structure.